The discretized tracer population balance equations (DTPBE) for aggregation problems is discussed and modified. It is shown that the original version was not entirely consistent with the associated discretized population balance equation (DPBE). These inconsistencies are remedied in a new formulation that retains the advantages of the original DTPBE, such as conservation of total tracer mass, prediction of tracer-weighted mean particle volume, and so on. Furthermore, the DTBE has been extended to an adjustable discretization. Numerous comparisons are made of the validity of the extended and modified formulation. (c) 2005 American Institute of Chemical Engineers